Wednesday, June 23, 2010

Accessibility: Advanced

fMRI experiments, with their small sample sizes, can easily fall victim to variability within the subject pool. This is especially true for patient studies. So it’s nice to step back and look at the big picture once in a while, and see where different studies agree and disagree.

Richlan and colleagues recently did meta-analysis of dyslexia brain imaging studies. They used an algorithm called Activation Likelihood Estimation (ALE), which models foci of activation as Gaussian probability distributions. (The software is called GingleALE. Ha.)

Richlan and colleagues picked studies with the following criteria:
1) Uses words, pseudowords or single letters as stimuli
2) Uses reading or reading related task in the scanner, and
3) Group comparisons are done in a standard stereotactic space.
The studies included PET and fMRI studies.

The take away message is that people with dyslexia underactivate posterior reading regions and may overactivate frontal regions.

The authors found underactivation in regions associated with the phonological reading pathway (reading by sounding out words), including the superior temporal gyrus and inferior parietal lobule. Interestingly, they found no difference in the angular gyrus, a region that has often been reported to be important to reading.

They also found underactivation the pathway associated with automatic whole word reading, including the left fusiform, inferior temporal and middle temporal regions.

At a less conservative threshold, the authors found that people with dyslexia overactivated the left inferor frontal region. This is typically interpreted as frontal regions being brought in to compensate for posterior reading regions.

They did find one posterior region as well that was overactivated in people with dyslexia : the left lingual gyrus, a lower level visual region. Perhaps again, a case of compensation.

All in all, a nice summary of dyslexia results. Again, I wonder about the relative variablility of dyslexics and controls, and how they affected the results.

4
comments:

I find it interesting that a meta-analysis reads like the conclusion section of a paper. Are experiments becoming so big and complex that we need division of labor -- the government funds hypotheses, research groups execute the methods, and others analyze the results? :)

I'm personally and professionally curious about any speculation you might have as to why those regions would be under-activated (lack of input, inability to process it, or what?) and what are the more-activated regions doing to compensate?

I work with adult students who often have some form of dyslexia. The problems they face are often not only with the dyslexia but also with the strategies, useful or not, for dealing with the situation. The increased activation of the left inferior frontal region certainly makes sense, but I would also wonder what fMRIs and PETs would show when adults engage in reading but rely on different coping strategies.